PTFE (polytetrafluoroethylene) is an advantageous material for a variety of purposes. The material is used in a wide variety of areas especially in its microporous form, for example, as expanded PTFE (ePTFE), e.g., in clothing articles, packings, filter membranes, medical implants, etc.
It is known that PTFE can be provided with fillers to obtain specific properties of this material. Fillers include particles of carbon, metal, metal oxides, glass or plastics. In this case the particle fillers are added to the PTFE starting material in the form of powder.
EP- B1-0 184 392 discloses structures and membranes, in which the pores of the finished structures are filled with solutions of polymers or monomers, that are then polymerized. However, these variants are confined to certain membranes or films, since a solvent that must fully wet the membranes must be used and the concentration of dissolved polymer or monomer must be kept very low. At high concentration of fillers the pores are clogged and the porosity is limited. In addition, these methods are limited to very open-pore membranes, since small-pore systems exhibit extremely reduced material transport after this treatment.
Filter membranes are a special example of application of ePTFE. The membranes used for industrial filters are typically cleaned at specific intervals. Scraping contamination from the surface of a filter membrane is included in this cleaning. In this example it is obvious that the membrane must be as abrasion-resistant as possible so that it withstands repeated mechanical cleaning.
Another example of a desired property of ePTFE is stability relative to energy rich radiation, e.g. gamma or beta radiation. Since ePTFE becomes brittle when exposed to energy rich radiation one can consider stabilizing the overall structure of filled ePTFE relative to the effect of energy rich radiation by selecting an appropriate filler in an appropriate amount.
The filler in ePTFE is generally found in the intermediate spaces of the porous ePTFE formed by the nodes and fibrils. For example, when filter membranes made of filled ePTFE are subject to mechanical stress, for example, by scraping off filter residues, part of the filler is released from the ePTFE structure on each exposure of the surface to mechanical stress, which ultimately adversely affects the properties of the material.